Hydrogen storage alloys possess the ability to reversibly absorb and desorb hydrogen during charge and discharge. A hydrogen storage alloy has a theoretical capacity density larger than that of cadmium, and unlike zinc electrodes, dendrites are not formed. For this reason, hydrogen storage alloys are considered as a promising material for negative electrodes of alkaline storage batteries.
In recent years, nickel-metal hydride storage batteries containing a hydrogen storage alloy are attracting attention also as power sources for driving electric vehicles or the like. For this reason, demand for nickel-metal hydride storage batteries having improved output characteristics and storage characteristics is growing.
For the negative electrodes of nickel-metal hydride storage batteries, hydrogen storage alloys having a CaCu5 type crystal structure are mainly used. For example, MmNi5 (where Mm represents a mixture of rare-earth elements) alloys, in which Ni is partially replaced by Co, Mn, Al, Cu or the like, are widely used.
A produced hydrogen storage alloy is subjected to an activation step, and then is used as an electrode alloy powder. Accordingly, many proposals are made regarding the activation step. One is, for example, an activation step in which hydrogen storage alloy powders containing nickel are immersed in an aqueous solution containing 30 to 80 wt % of sodium hydroxide and held at 90° C. or greater (see Patent Document 1).    Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-256301